Background

Managing authority of Queensland Motorways proposed to upgrade the existing Gateway Bridge River Crossing. The project included the duplication of the existing Gateway Bridge and the upgrade of 24 km of the Gateway Motorway from Miles Platting Road to Nudgee Road. There were virtually three projects in one: the Gateway Bridge duplication; a 16km motorway upgrade (south) and a new 7 km motorway deviation (north).

Problem / Task

Many of the constructions sat on the flood plain area of the Brisbane River which underlain mostly by soft and saturated fluvial deposits. Low bearing capacity and excessive settlement were highly likely to occur and consequently jeopardize the stability and safety of the constructed facilities.

Solution/ Design & Construction

To minimize the risks caused by the soft soils, ground improvement was extremely necessary. The designer adopted prefabricated vertical drains (PVD) to accelerate the dissipation of excess pore water in soft deposit and therefore to cause an increase in shear strength and a decrease in compressibility. The contractor first placed a layer of ACETex® PET woven geotextile, which was supplied by our Australian distributor Global Synthetics Pty Ltd, and then placed a layer of gravelly platform on the surface of the ground. ACETex® provided functions of reinforcement, separation, and filtration, so that the induced pore water could be easily dissipated without losing of fine particles. Then, PVDs were installed in square patterns (spaced 1.5 m center to center) to cover all the treatment areas. As a result of this method of accelerating the consolidation process, bearing failure and excessive settlement can be virtually eliminated.

Result

ACETex® PET has high tensile strength to reinforce soft soil layer, and also a unique structure that enhances its function as a filter fabric. It has both high permeability and the ability to restrict the movement of most soil particles, while allowing the very fine silts to flow into and out of the drain. With the use of ACETex®, the installation and the performance of PVDs have been greatly improved. The period of settlement and consolidation were shortened substantially; the stability and load carrying capacity were improved significantly. ACETex® together with PVDs will be beneficial for embankments constructed on soft ground for roadways, railroad tracks, or runways which must be functional in an extremely tight schedule.